User Forum of Software BASEMENT

sterrier

User

Registered: 2020-01-09

Posts: 4

Hydraulic time step

Dear all,

My Basement 2.8 R5771 model takes a long time to compute, and I’m looking into the hydraulic time step in order to understand why. In the “balance” output, the time step varies in the range 0.004 - 0.168 s. The output also gives the limiting element and edge. If I manually estimate the CFL number at that edge (velocity of ~0 m/s, water depth of ~50 m and edge length of ~20 m), it gives a maximum time step of 0.9 s according to equation 2.221 of the reference manual. So much larger than the one indicated in the balance output, which lets me think there might be a way to optimize the model.

I thought setting output_time_step = 0 for balance would show me the results for all time steps performed during a simulation to understand what is happening. But it doesn’t, it sets the hydraulic time step to 0.001 (default minimum_time_step I suppose). Then I discovered that changing the output time step (console or balance) can significantly change (double or half) the hydraulic time step. For example, in the tutorial files “Flaz-01-hydraulic-stationary”, changing the console time step from 100 to 10 changes the hydraulic time step in the balance output from 0.007 s to 0.024 s. However the total simulation time does not vary greatly, which raises doubts about what is the hydraulic time step actually used.

Is there a way to show the list of all hydraulic time steps performed during a simulation?
And is it normal that the output time step influences the hydraulic time step?

Best regards,

Stéphane